This invention relates to a novel method of solution mining potassium chloride from a subterranean deposit containing potassium chloride and sodium chloride. More particularly, this invention relates to a novel method whereby the salts are mined at a ratio near the invariant composition of the solution. And even more particularly, this invention involves a novel method of mining potassium chloride near the invariant composition of the solution at a temperature higher than the deposit temperature.
Potassium chloride usually occurs in mineral deposits closely associated with sodium chloride. Often, potassium chloride exists as mixtures or in combination with sodium chloride in the form of a potassium chloride-rich stratum (containing 15 to 60 percent potassium chloride based on the total weight of potassium chloride and sodium chloride in the stratum) or a plurality of such deposits which are disposed immediately above and below other strata which are lean as to potassium chloride (containing less than 15 percent thereof by weight) or which contain no substantial amount of potassium chloride and/or preponderantly sodium chloride. These strata may be folded or domed or take various other shapes and may contain various amounts of other minerals, such as clay, sulfates of calcium and magnesium, as well as chlorides of the alkaline earth metals and the like.
These deposits may be found anywhere between about 700 to between about 1800 meters deep and even deeper. Such deposits may be found in New Mexico, Utah, Northern United States, Canada as well as other parts of the world.
It is of commercial interest to produce potassium chloride from these subterranean deposits. A number of methods have been developed for the recovery of the potassium chloride by extraction with water. Accordingly, a well is drilled through the potassium chloride-rich strata or deposit and downwardly into the zone in which the potassium chloride concentration is low, i.e., below 15 percent based the weight of potassium chloride and sodium chloride, or is substantially non-existent and where sodium chloride is comparatively high. Water or an aqueous solution which is unsaturated as to sodium chloride is cause to flow down the well either through a pipe disposed in the well or through the concentric area within the well but outside the pipe, and the potassium chloride-lean, sodium chloride-rich strata is extracted to establish a cavity in the manner well known to the art of extracting sodium chloride from subterranean deposits. Two such cavities are created and joined by lateral growth. Subsequently, the cavity roof is mined upwardly through the potassium chloride-rich strata. In other instances two spaced wells are drilled through the potassium chloride rich strata or deposit and downwardly into the zone in which the potassium chloride concentration is low and communication between the bottom of the wells are made by fracturing with hydraulic pressure. Again, the fracture is mined upwardly through the potassium chloride-rich strata. There are other recognized methods of the art as well. In all these cases a large cavity is best created by circulating water or brine through one well and withdrawing a solution enriched in sodium chloride and potassium chloride from a second well. In many of these methods potassium chloride is mined at a ratio very close to the ratio at which it occurs in the deposit.
In one method taught by U.S. Pat. No. 3,278,234 solution mining is enhanced when dissolving potassium chloride and sodium chloride from the deposit to form an aqueous solution richer in both potassium chloride and sodium chloride by maintaining the temperature of the solvent hotter than the effluent and maintaining the temperature within about 15.degree. C. of the natural (undisturbed) formation temperature thereby avoiding undue cooling of the cavity solution. This patent further discloses that at depths below about 700 meters to 900 meters the effluent temperature is held within about 5.degree. C. and preferably 3.degree. C. of the natural deposit temperature.
In another method taught by U.S. Pat. No. 3,148,000, potassium chloride is mined in commercial amounts from a deposit containing from 0.1 to 15 percent or more of potassium chloride based up the combined amount of potassium chloride and sodium chloride within the deposit. The cavity is increased in size by reducing the water pressure therein thereby causing its roof to collapse. Similarly, a rubble of deposit ore may be created in the center of the cavity by drilling a well to immediately above the top of the cavity and exerting hydraulic pressure on the top of the cavity to cause the roof thereof to fall into the cavity. (See U.S. Pat. No. 2,919,909). In this fashion, a large surface area of potassium chloride-rich ore is provided within the cavity. Thus, the ratio of potassium chloride recovered from the cavity is greater than the ratio that occurs in the ore deposit.
In the cases where water is used as the solvent and a large cavity is created as in the former methods described, potassium chloride is dissolved at the ratio it occurs in the deposit, e.g., about 30 percent potassium chloride by weight. In the method of rubble mining, the invariant composition of the solution may be reached for the deposit temperature, e.g., a composition where the solution attains a ratio greater than that which occurs in the deposit, e.g., greater than a deposit ratio of 30 percent potassium chloride. By invariant composition is meant a solution having a composition saturated with respect to both potassium chloride and sodium chloride at a given temperature (this composition ratio is changed by the presence of other metal ions such as magnesium and calcium).
In yet another method taught in U.S. Pat. No. 3,262,741 a solution which is relatively saturated with respect to sodium chloride and relatively unsaturated with respect to potassium chloride and which is produced from a new cavity is fed into a fully developed cavity wherein the solution is allowed to remain until it reaches the invariant composition for the deposit temperature. The resident time for the relatively unsaturated solution is reduced by feeding the relatively unsaturated solution produced from the new cavity into a cavity that is still productive. But, the average flow through a cavity mined in this manner is much less than that which is allowed by average size piping, e.g., less than about 90 cubic meters per hour.
Hence, in all of the above described methods no provision has been made so that an invariant composition of the cavity solution may be attained at the highest temperature and especially above the natural deposit temperature. The prior art, on the contrary, has taught that solution mining best occur as near the deposit temperature as possible. It is therefore desired that a greater amount of potassium chloride be produced from a subterranean formation whereby the cavity solution attains the invariant composition at the highest temperature and even higher than the natural formation temperature so long as an appreciable amount of heat is not irretrievably lost into the formation. It is also desired that the solution be produced from the cavity at a rate allowable by average piping size, e.g., about 90 cubic meters per hour or greater.